1. Field of the Invention
This invention relates to electrical power distribution transformers. In particular the invention relates to methods of making such distribution transformers of the type which include a core and coil assembly having a wound magnetic core with a central window and one or more electric coils which extend through said core window, and to the transformers so made.
2. Description of the Related Art
Two known methods of making a transformer core and coil assembly of the above-defined type, and in which the core is of overall rectangular shape, will now be described.
In the first such known method the wound core is made by winding magnetic steel strip of single width into a circular roll, and in winding each turn it is cut at approximately the same point. The circular roll is then pressed into an overall rectangular shape core having distributed gaps through one side of the rectangle where the turns were cut, and it is then annealed to fix the rectangular shape. The cut core turns are then opened up and bent out to form a U-shape, a pre-formed rectangular cylindrical coil is assembled on each of the two legs of the U-shape, and the cut core turns are then closed to re-form the rectangular core shape and are jointed. However well the cuts are jointed they will add significantly to the power loss of the core. Also with this method, the machinery for cutting the magnetic steel strip involves significant cost. Futhermore the present and expected future trend is to use progressively thinner magnetic steel strip which has inherently lower power loss, but thinner strip is more difficult to handle in processes which involve cutting. Another disadvantage of this method is that the equipment and process involved in annealing the core contributes significantly to the cost of manufacturing the transformer.
In the second known method of making a rectangular shape wound core transformer, magnetic steel strip of varying width is wound continuously without cuts on to a rectangular mandrel to form an overall rectangular shape core with an approximately circular cross-section. The core is then annealed to fix the rectangular shape. Split mandrels are then fitted over two legs of the core and a circular cylindrical coil is wound on to each mandrel. This second method avoids the manufacturing and power loss disadvantages associated with cutting in the above-described first method. However there is still the cost disadvantage of annealing the core. There are two further disadvantages of this second method. Firstly the only approximately circular cross-section of the core within the circular coils gives a significant reduction in space factor and hence higher power loss. Secondly, for larger size coils there is an increased level of difficulty in winding the coils leading to a practical upper limit of approximately 50 KVA rated power for transformers made by this method, which does not cover the full rated power range required for distribution transformers.
Conventionally, rectangular shape wound transformer cores, whether cut or uncut, have been made with non-amorphous steel strip. More recently such transformer cores have become known which are made with amorphous steel strip. This material has much lower power loss than non-amorphous steel, but this advantage is partially offset by the higher intrinsic material cost. Also, amorphous steel has only been available with a strip width up to approximately 200 mm, 213 mm being the highest strip width of which we are aware, which limits the size of wound cores using a single strip width and hence the rated power of transformers using such cores so that they do not cover the full rated power range required for distribution transformers.